1. Field of the Invention
The present invention relates to a charged particle transfer mask designed to form on a sensitive substrate a fine high density pattern that is less than 100 nm in size with high throughput and accuracy. In particular, the present invention relates to a charged particle transfer mask which corrects the de-focus caused by the Coulomb effect which occur when the pattern density is non-uniform and a pattern distortion caused by the Coulomb effect.
2. Description of the Related Art
A conventional electron beam exposure system irradiates an electron beam onto a mask (reticle) having a circuit pattern of one entire semiconductor chip, and reduce-transfers the image of the pattern within the irradiation range using 2-stage projection lenses. In this case, when an attempt is made to transfer the pattern at once by batch-irradiating the electron beam onto the entire range of the mask, the pattern cannot be transferred accurately. A division transfer system capable of transferring a pattern more accurately than this conventional system is being proposed.
An apparatus compatible with this division transfer system which forms a reduced image of the mask on a sensitive substrate mounted on a stage by dividing the mask into small regions (subfields) and moving or continuously scanning an irradiation beam for each of the subfields based on the step-and-repeat system is known from the prior art.
Also, another method of forming a reduced image of a mask onto a sensitive substrate is known. In this method, an electron beam is irradiated onto a mask having a membrane on which a pattern is formed with a scattering body or an electron beam is irradiated onto such a mask whose scattering body has holes. The beam scattered with the scattering body is then removed at a contrast aperture.
When the current supplied to the beam is increased for the conventional mask so as to increase the throughput, the beam is blurred or the focus position of the beam is displaced by the Coulomb effect (hereafter this state will be called the state of "de-focus"), and the image is distorted. Moreover, in a subfields on which the pattern density is non-uniform, the degree of the blur of the beam or the amount of distortion or de-focus is increased. Here, the image distortion refers to the entire displacement from the set values for the shape of the image, including the displacement in the rotation angle of the image and the displacement in the magnification factor.
For example, when the pattern density (the electric current amount) differs from one subfields to another in the mask, the amount of de-focus caused by the Coulomb effect differs from one subfields to another accordingly. Therefore, the height (position) of the focal point plane differs from one sub-field to another. In such a case, the height of the sensitive substrate needs to be adjusted by moving the stage in the height direction or re-focusing the electronic optical system.
Moreover, when the electron beam is irradiated onto the mask, the current distribution generated inside the subfields becomes non-uniform. This non-uniformly distributed current creates a large non-uniform distortion on the transferred pattern. Here, the non-uniform pattern distortion refers to, for example, a distortion type that transfers a relatively long straight line pattern on the mask onto a curve of high order. It is very hard to correct such a distortion using the electronic optical system.
As said above, in general, the de-focus, magnification error (displacement), rotation error, orthogonal error (square pattern to parallelogram) and non-linear distortion (as shown in FIG. 10) are caused by Coulomb effect. These errors are generated at a large beam current. Especially large non-linear distortion is generated by non-uniform pattern distribution in the subfields.
Given these problems, it is an object of the present invention to provide a charged particle beam transfer mask in which the differences in the amount of de-focus caused by Coulomb effect, which are generated when the pattern density is non-uniform from one sub-field to another, are corrected. It is also an object of the present invention to provide a charged particle beam transfer mask in which the pattern distortion caused by the Coulomb effect is corrected.